Current driver with output current clamping

ABSTRACT

In one aspect, a current driver, includes an operational amplifier that includes a first input port configured to receive a reference signal and a second input port configured to receive a variable signal. The variable signal is a function of an output current of the current driver. The reference signal corresponds to a selected maximum output current of the current driver. The current driver also includes a feedback transistor comprising a gate coupled to the output of the operational amplifier and a summing junction coupled to a drain of the feedback transistor and configured to receive a signal from the drain to enable clamping of the output current of the current driver to the maximum output current when the variable signal exceeds the reference signal. The summing junction is coupled to a set of transistors configured to provide the output current of the current driver.

BACKGROUND

Cellular phones, cameras, computers and tablet computers includeelectronic modules such as, for example, a camera module. Theseelectronic modules generally have a power budget. Exceeding the powerbudget often has undesirable consequences such as quickly reducingbattery life or inhibiting functionality in other electronic modules.Sometimes these electronic modules include power limitations such as,for example, an average current being drawn and a maximum current beingdrawn. If, for example, an electronic module exceeded its maximumcurrent budget, a circuit may be used to reduce the current being drawn.Often this is performed by reducing the power supply to the electronicmodule which has undesirable consequences such as disabling theelectronic module.

SUMMARY

In one aspect, a current driver, includes an operational amplifier thatincludes a first input port configured to receive a reference signal anda second input port configured to receive a variable signal. Thevariable signal is a function of an output current of the currentdriver. The reference signal corresponds to a selected maximum outputcurrent of the current driver. The current driver also includes afeedback transistor comprising a gate coupled to the output of theoperational amplifier and a summing junction coupled to a drain of thefeedback transistor and configured to receive a signal from the drain toenable clamping of the output current of the current driver to themaximum output current when the variable signal exceeds the referencesignal. The summing junction is coupled to a set of transistorsconfigured to provide the output current of the current driver.

In another aspect, a current driver includes an operational amplifierthat includes a first input port configured to receive a referencesignal and a second input port configured to receive a variable signal.The variable signal is a function of an output current of the currentdriver. The reference signal corresponds to a selected maximum outputcurrent of the current driver. The current driver also includes afeedback transistor comprising a gate coupled to the output of theoperational amplifier and a summing junction coupled to a drain of thefeedback transistor and configured to receive a signal from the drain toenable clamping of the output current of the current driver to themaximum output current when the variable signal exceeds the referencesignal. The summing junction is coupled to an H-Bridge circuit. Thecurrent driver further includes a damping resistor coupled between thesumming junction and the H-bridge circuit, the H-bridge circuitconfigured to provide the output current of the current driver andcoupled to an actuator coil and a digital-to-analog converter (DAC)coupled to the feedback transistor and configured to provide thereference voltage.

In a further aspect, a current driver includes a first operationalamplifier that includes a first input port configured to receive areference signal and a second input port configured to receive avariable signal. The variable signal is proportional to an outputcurrent of the current driver. The reference signal corresponds to aselected maximum output current of the current driver. The currentdriver also includes a first feedback transistor that includes a gatecoupled to the output of the first operational amplifier, a summingjunction coupled to a drain of the first feedback transistor andconfigured to receive a signal from the drain to enable clamping of theoutput current of the current driver to the maximum output current whenthe variable signal exceeds the reference signal. The summing junctionis coupled to an H-Bridge circuit. The current driver further includes adamping resistor coupled between the summing junction and the H-bridgecircuit, the H-bridge circuit configured to provide the output currentof the current driver and coupled to an actuator coil of a camera modulehaving autofocusing functionality, a transconductance amplifierconfigured to receive an input signal to the current driver, a currentgain stage circuit that includes the summing junction and coupled to anoutput of the transconductance amplifier, a digital-to-analog converter(DAC) coupled to the first feedback transistor and configured to providethe reference voltage, a second operational amplifier comprising a firstinput coupled to the H-bridge circuit and a second feedback transistorcomprising a gate coupled to the output of the second operationalamplifier, a drain coupled to an input of the first operationalamplifier and a source coupled to the first input of the secondoperational amplifier.

DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention, as well as the invention itselfmay be more fully understood from the following detailed description ofthe drawings, in which:

FIG. 1 is a functional block diagram of a circuit that includes acurrent driver;

FIG. 2 is a block diagram of an example of the current driver and anexample representation of a load being driven by the current driver;

FIG. 3 is a graph of an example of percentage of full scale currentversus voltage curve; and

FIG. 4 is a graph of an example of percentage of full scale currentversus different codes entered into an 5-bit DAC to set a maximum outputcurrent of the driver.

DETAIL DESCRIPTION

Described herein is a circuit that provides trim using a clamp. In oneexample, this to circuit is a closed loop bi-directional clamp circuitor unidirectional clamp circuit, which may be used, for example, as adriver of a voice coil motor camera module in a cellular phone and, inparticular, camera autofocusing applications. In one particular example,the clamp circuit prevents the camera module from exceeding theallowable, transient current consumption of the camera module due to thelimits of the power supply. While an example of the voice coil of amotor camera module is described herein, one of ordinary skill in theart would appreciate that the techniques described herein may be used inany resistive and/or inductive load that requires trim.

Referring to FIG. 1, a circuit 10 includes an output driver control 12,a current driver 20 and an actuator coil 24. The current driver 20includes an H-bridge circuit that includes two p-metal oxidesemiconductor field effect transistors (pMOSFETs) 42 a, 42 b and twon-metal oxide semiconductor field effect transistors (nMOSFETs) 44 a, 44b. The current driver 20 includes a terminal M+ and a terminal M−.

In one example, the actuator coil 24 drives a ferromagnetic materialsuch as a hard ferromagnetic material as in a permanent magnet or a softferromagnetic material as in examples where the device may only pull onthe soft ferromagnetic material. In other examples, the actuator coil 24drives or moves a lens assembly with a ferromagnetic material.

The output driver control 12 is coupled to the gates of each of thepMOSFETs 42 a, 42 b and nMOSFETs 44 a, 44 b and is configured to providea signal to each of the MOSFET gates to control the current beingprovided by the current driver 20. The current driver 20 provides acurrent to one of the M+ terminal and the M− terminal depending on whichMOSFETs 42 a, 42 b, 44 a, 44 b are activated. In one particular example,when the pMOSFET 42 a and the nMOSFET 44 b are turned on, the currentfrom a power supply VCC goes through the pMOSFET 42 a to the M+ terminalthrough the actuator coil, out the M− terminal and through the nMOSFET44 b to ground. In another example, when the pMOSFET 42 b and thenMOSFET 44 a are turned on, a current from the power supply VCC goesthrough the pMOSFET 42 b out the M− terminal, in the M+ terminal andthrough the nMOSFET 44 a to ground. As will be further described herein,the current driver 20 has a clamping function that limits the currentoutput of the current driver to a selected maximum output current.

Referring to FIG. 2, one particular example of a current driver 20 is acurrent driver 20′. The current driver 20 is coupled to and providescurrent to a load (i.e., an actuator coil 24′). The current driver 20′includes a transconductance amplifier 102 with the gain inverselyproportional 20 to resistance of R1. The transconductance amplifier 102is supplied with an input signal Vind, which is also the input to thecurrent driver 20′. The transconductance amplifier transconductanceamplifier 102 represents a first stage with a gain, GM1. In oneparticular example, GM1 is equal to 4/R1. The output of thetransconductance amplifier 102 is coupled to a current gain stage 106.

The current gain stage 106 includes a summing junction 110, which addsan output signal from the transconductance amplifier 102 and subtractsan ICLAMP signal and a feedback signal ifb. A resultant signal from thesumming junction 110 is supplied to a damping resistor R4. The currentgain stage 106 represents a second stage. In one particular example, ifthe current gain A2 of the current gain stage 106 is 10 the total gainacross the first and second stages is equal to GM1 times A2 or GM1 times10.

The damping resistor R4 is coupled to a gate of the transistor M1 whichis coupled to a transistor M2. The drain of the transistor M1 is coupledto an actuator coil 24′ represented by a resistive load RCOIL and aninductive load LCOIL. In one example, the M1 is a pMOSFET used in anH-bridge circuit such as pMOSFETs 42 a or 42 b (FIG. 1). A capacitor C(e.g., a parasitic Miller capacitor) couples the gate of the transistorM1 and the resistor R4 to the drain of the transistor M1.

The drain of the transistor M2 is coupled to a gate of a transistor M3and provides the feedback signal ifb to the summing junction 110. Thetransistors M1 and M3 are current mirrors of the transistor M2. In oneexample, the transistors M1 and M3 are pMOSFETs. The transistors M1, M2,M3 represent a third stage. In one particular example, if the gainacross the third stage current gain A3 is 60, then the gain across thethree stages is equal to GM1 times A2 times A3 or GM1 times A2 times 60.In another particular example, if R1 is equal to 20 K ohms, A2 is equalto 10 and A3 is equal to 60 then the actual value of the overall gain isequal to 100 mA/V.

In order to keep the gain of the third stage substantially constant, adrain matching mechanism is used. The drain matching mechanism includesan op amp 112 with an output coupled to a transistor M5. One input ofthe op amp 112 is coupled to the actuator coil 24′ represented by theresistive load RCOIL and the inductive load LCOIL. The other input ofthe op amp 112 is connected to the drain of the transistor M3 and to thesource of the transistor M5. The purpose of the drain matching mechanismis to generate an accurate feedback current proportional to the outputcurrent of the current driver 20′, which is the current over theactuator coil. The current through transistor M5 is scaled to 1/A3. Inone example, the transistor M5 is a pMOSFET.

The drain of the transistor M5 is coupled to a master feedbackconfiguration that includes an op amp 122, a transistor M4 and a currentdigital-to-analog 130 (DAC). The op amp 122 receives at a first inputport a voltage at a point X, voltage X. The point X is coupled toresistor R2 coupled to ground or some other common mode referencevoltage and to the drain of the M5 transistor. The voltage X is avariable signal that is proportional to the output of the current driver20′. In one example, the variable signal is linearly proportional to theoutput of the current driver 20′. In other examples, the variable signalis non-linearly proportional to the output of the current driver 20′,for example, if there is no drain matching mechanism.

The op amp 122 receives at a second input port a signal VREF generatedfrom the output of the current DAC 130. The output of the op amp 122 iscoupled to a gate of the transistor M4. The source of the transistor M4is coupled to the power supply VCC. The drain of the transistor M4 iscoupled to the summing junction 110 and when the transistor M4 is tunedon, the ICLAMP signal is provided to the summing junction 110.

A current IREF is provided to the current DAC 130. The output of thecurrent DAC 130 is coupled to ground or another reference voltage by aresistor R3. In one example, the current DAC 130 is an n-bit DAC, wheren is an integer greater than zero. In one example, n is greater than orequal to three and less than or equal to nine. In one particularexample, n is equal to five. A user can use coded bits to control theoutput of the current DAC 130 and set the VREF signal. The VREF signalcorresponds to a maximum output current of the current driver 20′.

If the voltage X remains below the signal VREF, the op amp 122 does notturn on the transistor M4. However, if the voltage X exceeds the signalVREF (i.e., indicating that the current driver will exceed the maximumallowable current), then the transistor M4 is turned on and the ICLAMPsignal is sent to the summing junction 110 which will enable a reductionin the signal received from the current gain stage 106 and force thecurrent driver 20′ to clamp the output signal substantially at themaximum allowable current.

Drain matching described previously makes the actuator coil part of themaster feedback loop. The inductance of the actuator represented byLCOIL and the parasitic capacitance of the transistors M1, M2, M3 forman oscillator circuit which is compensated by the damping resistor R4.

It will be appreciated by one of ordinary skill in the art that currentdriver 20′ in FIG. 2 is a simplified unidirectional representation ofthe current driver 20 of FIG. 1 and as such not all elements may benecessarily depicted. For example, nMOSFETs in the H-bridge are notdepicted in FIG. 2. Also, one of ordinary skill in the art wouldappreciate that FIG. 2 could be implemented using nMOSFETs in theH-bridge instead of pMOSFETs by simply inverting the diagram.

In other examples, the ground shown in FIG. 2 may be replaced by anegative potential for example.

Referring to FIG. 3, a graph 300 illustrates the clamping function ofthe current driver 20′. As the input voltage, Vind, to the currentdriver 20′ increases the clamping function is not turned on. Forexample, in the region 302 of the curve the output current increaseslinearly with increasing input voltage Vind while the output current isbelow a maximum selected output current. When the output current reachesthe maximum selected output current 308, the clamping function isengaged and the output current of the current driver 20′ remainssubstantially constant at the maximum selected output current withincreasing input voltage Vind.

Referring to FIG. 4, a graph 400 of an example of current versusdifferent codes that may be entered into a n-bit DAC to set a maximumoutput current of the current driver 20′. By selecting a code for theDAC 130, a user is able to control the VREF signal and thereby select amaximum output current provided by the current driver 20′. The graph 400in FIG. 4 follows a two's complement behavior. For example, the lowestmaximum output current starts at code 16 and continues to increase tocode 31. The maximum output current continues to increase starting atcode 0. In this example, codes 7 to 15 remain flat at about 84.6% offull scale due to limitations in the power supply, VCC. Using otherpower supplies can increase the maximum output current of 84.6% of fullscale to higher values. By using the DAC 130 a user can adjust themaximum output current of the current driver despite variations inprocessing of the circuit 20′, supply voltage characteristics andtemperature.

The elements described herein are not limited to the specific examplesdescribed. For example, one or more of the operational amplifiers 112,122 may be an operational transconductance amplifier (OTA). In oneexample, the transistors M4 and M5 are pMOSFETs. In another example, thecircuit 20′ may be reconfigured using nMOSFETs rather than pMOSFETs. Ina further example, the drain mechanism in FIG. 2 may be removed byremoving the op amp 112 and the transistor M5 and directly coupling thedrain of the transistor M3 to the point X.

One of ordinary skill in the art would appreciate that the MOSFETs maybe replaced by other devices such as, for example, a bipolar junctiontransistor (BJT) or a double-diffused metal-oxide-semiconductor (DMOS).

Elements of different embodiments described herein may be combined toform other embodiments not specifically set forth above. Otherembodiments not specifically described herein are also within the scopeof the following claims:

What is claimed is:
 1. A current driver, comprising: an operationalamplifier comprising a first input port configured to receive areference signal and a second input port configured to receive avariable signal, the variable signal being a function of an outputcurrent of the current driver, the reference signal corresponding to aselected maximum output current of the current driver; a feedbacktransistor comprising a gate coupled to the output of the operationalamplifier; and a summing junction coupled to a drain of the feedbacktransistor and configured to receive a signal from the drain to enableclamping of the output current of the current driver to the maximumoutput current when the variable signal exceeds the reference signal,the summing junction being coupled to a set of transistors configured toprovide the output current of the current driver, wherein at least oneof the set of transistors is a mirror of another one of the set oftransistors.
 2. A current driver, comprising: an operational amplifiercomprising a first input port configured to receive a reference signaland a second input port configured to receive a variable signal, thevariable signal being a function of an output current of the currentdriver, the reference signal corresponding to a selected maximum outputcurrent of the current driver; a feedback transistor comprising a gatecoupled to the output of the operational amplifier; a summing junctioncoupled to a drain of the feedback transistor and configured to receivea signal from the drain to enable clamping of the output current of thecurrent driver to the maximum output current when the variable signalexceeds the reference signal, the summing junction being coupled to aset of transistors configured to provide the output current of thecurrent driver; and an n-bit current digital-to-analog converter (DAC)coupled to the feedback transistor and configured to provide thereference voltage, wherein is n is an integer greater than zero.
 3. Thecircuit of claim 2 wherein n is greater than or equal to three and lessthan or equal to nine.
 4. The circuit of claim 3 wherein n is equal tofive.
 5. The circuit of claim 1, further comprising a resistor coupledbetween the summing junction and the set of transistors.
 6. A currentdriver, comprising: an operational amplifier comprising a first inputport configured to receive a reference signal and a second input portconfigured to receive a variable signal, the variable signal being afunction of an output current of the current driver, the referencesignal corresponding to a selected maximum output current of the currentdriver: a feedback transistor comprising a gate coupled to the output ofthe operational amplifier; a summing junction coupled to a drain of thefeedback transistor and configured to receive a signal from the drain toenable clamping of the output current of the current driver to themaximum output current when the variable signal exceeds the referencesignal, the summing junction being coupled to a set of transistorsconfigured to provide the output current of the current driver; atransconductance amplifier configured to receive an input signal to thecurrent driver; and a current gain stage circuit comprising the summingjunction and coupled to an output of the transconductance amplifier. 7.The circuit of claim 6, wherein the operational amplifier is a firstoperational amplifier and the feedback transistor is a first feedbacktransistor, and further comprising: a second operational amplifiercomprising a first input coupled to the set of transistors; and a secondfeedback transistor comprising a gate coupled to the output of thesecond operational amplifier, a drain coupled to an input of the firstoperational amplifier and a source coupled to the first input of thesecond operational amplifier.
 8. The circuit of claim 7 wherein at leastone of the first and second operational amplifiers is an operationaltransconductance amplifier.
 9. The circuit of claim 1 wherein each ofthe set of transistors is one of nMOSFETs or pMOSFETs.
 10. The circuitof claim 1 wherein at least one two of the set of transistors are partof an H-bridge circuit.
 11. (canceled)
 12. A current driver, comprising:an operational amplifier comprising a first input port configured toreceive a reference signal and a second input port configured to receivea variable signal, the variable signal being a function of an outputcurrent of the current driver, the reference signal corresponding to aselected maximum output current of the current driver; a feedbacktransistor comprising a gate coupled to the output of the operationalamplifier; and a summing junction coupled to a drain of the feedbacktransistor and configured to receive a signal from the drain to enableclamping of the output current of the current driver to the maximumoutput current when the variable signal exceeds the reference signal,the summing junction being coupled to a set of transistors configured toprovide the output current of the current driver, wherein the variablevoltage corresponds to the output current of the current driver providedto a load.
 13. The circuit of claim 12 wherein the variable voltage isproportional to the output current of the current driver provided to aload.
 14. The circuit of claim 13 wherein the variable voltage islinearly proportional to the output current of the current driverprovided to a load.
 15. The circuit of claim 12 wherein the load is anactuator coil for a camera module.
 16. The circuit of claim 15 whereinthe camera module comprises autofocusing functionality.
 17. A currentdriver, comprising: an operational amplifier comprising a first inputport configured to receive a reference signal and a second input portconfigured to receive a variable signal, the variable signal being afunction of an output current of the current driver, the referencesignal corresponding to a selected maximum output current of the currentdriver, a feedback transistor comprising a gate coupled to the output ofthe operational amplifier; a summing junction coupled to a drain of thefeedback transistor and configured to receive a signal from the drain toenable clamping of the output current of the current driver to themaximum output current when the variable signal exceeds the referencesignal, the summing junction being coupled to an H-Bridge circuit; adamping resistor coupled between the summing junction and the H-bridgecircuit; the H-bridge circuit configured to provide the output currentof the current driver and coupled to an actuator coil; and adigital-to-analog converter (DAC) coupled to the feedback transistor andconfigured to provide the reference voltage.
 18. The circuit of claim 17wherein the variable voltage is proportional to the output current ofthe current driver provided to the actuator coil.
 19. The circuit ofclaim 18, further comprising: a transconductance amplifier configured toreceive an input signal to the current driver, and a current gain stagecircuit comprising the summing junction and coupled to an output of thetransconductance amplifier.
 20. The circuit of claim 19, wherein theoperational amplifier is a first operational amplifier and the feedbacktransistor is a first feedback transistor, and further comprising: asecond operational amplifier comprising a first input coupled to the setof transistors; and a second feedback transistor comprising a gatecoupled to the output of the second operational amplifier, a draincoupled to an input of the first operational amplifier and a sourcecoupled to the first input of the second operational amplifier.
 21. Acurrent driver, comprising: a first operational amplifier comprising afirst input port configured to receive a reference signal and a secondinput port configured to receive a variable signal, the variable signalbeing proportional to an output current of the current driver, thereference signal corresponding to a selected maximum output current ofthe current driver, a first feedback transistor comprising a gatecoupled to the output of the first operational amplifier, a summingjunction coupled to a drain of the first feedback transistor andconfigured to receive a signal from the drain to enable clamping of theoutput current of the current driver to the maximum output current whenthe variable signal exceeds the reference signal, the summing junctionbeing coupled to an H-Bridge circuit; a damping resistor coupled betweenthe summing junction and the H-bridge circuit; the H-bridge circuitconfigured to provide the output current of the current driver andcoupled to an actuator coil of a camera module having autofocusingfunctionality; a transconductance amplifier configured to receive aninput signal to the current driver, a current gain stage circuitcomprising the summing junction and coupled to an output of thetransconductance amplifier; a digital-to-analog converter (DAC) coupledto the first feedback transistor and configured to provide the referencevoltage; a second operational amplifier comprising a first input coupledto the H-bridge circuit; and a second feedback transistor comprising agate coupled to the output of the second operational amplifier, a draincoupled to an input of the first operational amplifier and a sourcecoupled to the first input of the second operational amplifier.